This invention relates generally to semiconductor manufacture, and more particularly to a method for forming a transparent window in a polishing pad for use in chemical mechanical polishing (CMP).
In the process of fabricating modem semiconductor integrated circuits (ICs), it is necessary to form various material layers and structures over previously formed layers and structures. However, the prior formations often leave the top surface topography of an inprocess wafer highly irregular, with bumps, areas of unequal elevation, troughs, trenches, and/or other surface irregularities. These irregularities cause problems when forming the next layer. For example, when printing a photolithographic pattern having small geometries over previously formed layers, a very shallow depth of focus is required. Accordingly, it becomes essential to have a flat and planar surface, otherwise, some parts of the pattern will be in focus and other parts will not. In fact, surface variations on the order of less than 1000 xc3x85 over a 25xc3x9725 mm die would be preferable. In addition, if the irregularities are not leveled at each major processing step, the surface topography of the wafer can become even more irregular, causing further problems as the layers stack up during further processing. Depending on the die type and the size of the geometries involved, the surface irregularities can lead to poor yield and device performance. Consequently, it is desirable to effect some type of planarization, or leveling, of the IC structures. In fact, most high density IC fabrication techniques make use of some method to form a planarized wafer surface at critical points in the manufacturing process.
One method for achieving semiconductor wafer planarization or topography removal is the chemical mechanical polishing (CMP) process. In general, the chemical mechanical polishing (CMP) process involves holding and/or rotating the wafer against a rotating polishing platen under a controlled pressure. As shown in FIG. 1, a typical CMP apparatus 10 includes a polishing head 12 for holding the semiconductor wafer 14 against the polishing platen 16. The polishing platen 16 is covered with a pad 18. This pad 18 typically has a backing layer 20 which interfaces with the surface of the platen and a covering layer 22 which is used in conjunction with a chemical polishing slurry to polish the wafer 14. However, some pads have only a covering layer and no backing layer. The covering layer 22 is usually either an open cell foamed polyurethane (e.g. Rodel IC1000) or a sheet of polyurethane with a grooved surface (e.g. Rodel EX2000). The pad material is wetted with the chemical polishing slurry containing both an abrasive and chemicals. One typical chemical slurry includes KOH (Potassium Hydroxide) and fumed-silica particles. The platen is usually rotated about its central axis 24. In addition, the polishing head is usually rotated about its central axis 26, and translated across the surface of the platen 16 via a translation arm 28. Although just one polishing head is shown in FIG. 1, CMP devices typically have more than one of these heads spaced circumferentially around the polishing platen.
A particular problem encountered during a CMP process is in the determination that a part has been planarized to a desired flatness or relative thickness. In general, there is a need to detect when the desired surface characteristics or planar condition has been reached. This has been accomplished in a variety of ways. Early on, it was not possible to monitor the characteristics of the wafer during the CMP process. Typically, the wafer was removed from the CMP apparatus and examined elsewhere. If the wafer did not meet the desired specifications, it had to be reloaded into the CMP apparatus and reprocessed. This was a time consuming and labor-intensive procedure. Alternatively, the examination might have revealed that an excess amount of material had been removed, rendering the part unusable. There was, therefore, a need in the art for a device which could detect when the desired surface characteristics or thickness had been achieved, in-situ, during the CMP process.
Several devices and methods have been developed for the in-situ detection of endpoints during the CMP process. For instance, devices and methods that are associated with the use of ultrasonic sound waves, and with the detection of changes in mechanical resistance, electrical impedance, or wafer surface temperature, have been employed. These devices and methods rely on determining the thickness of the wafer or a layer thereof, and establishing a process endpoint, by monitoring the change in thickness. In the case where the surface layer of the wafer is being thinned, the change in thickness is used to determine when the surface layer has the desired depth. And, in the case of planarizing a patterned wafer with an irregular surface, the endpoint is determined by monitoring the change in thickness and knowing the approximate depth of the surface irregularities. When the change in thickness equals the depth of the irregularities, the CMP process is terminated. Although these devices and methods work reasonably well for the applications for which they were intended, there is still a need for systems which provide a more accurate determination of the endpoint.
One such system employs a CMP apparatus in which a hole is formed in a platen and the overlying platen pad. The hole is positioned so that it has a view of the wafer held by a polishing head during a portion of the platen""s rotation. A laser interferometer is fixed below the platen in a position enabling the laser beam projected by the laser interferometer to pass through the hole in the platen and strike the surface of the overlying wafer during the time when the hole is adjacent to the wafer. Various polishing pad embodiments include a transparent window in the pad. One of the concerns with the disclosed polishing pad arrangements is the leakage of slurry into the hole below the window of the polishing pad. This is a serious concern because any more than a trace amount of slurry will tend to scatter the light traveling through it, thus attenuating the laser beam emitted from the laser interferometer. The slurry leakage will thus cause inaccurate measurements with a laser interferometer, or even inoperability of the device.
In one method for detecting the end point in an in-situ polishing process, a platen is provided with a hole, or aperture, through which a laser interferometer is able to transmit laser light to the surface of the wafer being polished. The pad is configured with a transmissive portion that is positioned over the aperture and the rotatable platen. Thus, a relatively clear path to the wafer surface is provided by the combination of the platen and the pad. In one embodiment, the platen hole is formed with a stepped diameter to form a shoulder. A quartz insert is contained within the shoulder and functions as a window for the laser beam. The interface between the platen and the insert is sealed. The quartz insert protrudes above the top surface of the platen and partially into the platen pad in order to minimize the gap between the top surface of the insert and the surface of the wafer. This minimizes the amount of slurry trapped in the gap, thus reducing the attenuation of the laser beam emitted from the laser interferometer. It is desirable to make the gap as small as possible to reduce the amount of slurry in the gap. The fixing of the quartz insert within the platen is a concern, however since the wear of the pad could become so great that the top surface of the insert would touch the wafer and damage the wafer. In order to overcome this problem, another embodiment of the arrangement provides a polishing pad that has an integral window. For example, the window may be made of a polyurethane material that will not scratch the wafer and is co-planar with the top surface of the polishing pad. One of the disadvantages of the polishing pad provided with the integral window is the precise registration of the window pad over the aperture in the platen. The precision placement of the window over the aperture by an operator during the replacement of a pad may be time consuming and reduces overall production throughput. Also the polishing pads may be relatively expensive and more difficult to make since they contain a window that must be precisely inserted and fixed within the pad. Since the polishing pads are a major consumable item of the chemical mechanical polishing apparatus, this relatively more complex pad, which needs to be precisely assembled, will increase the operating cost of the apparatus.
There is a need for an arrangement in a chemical mechanical polishing apparatus in which a laser interferometer may be used to measure the condition of a wafer being polished, but reduces the cost of the individual polishing pads that are used.
This and other needs are met by an embodiment of the present invention which provides an arrangement for a polishing a workpiece in a chemical mechanical polishing apparatus. The arrangement includes a rotatable platen having a planar top surface, a bottom surface, and an aperture extending through the platen providing a transparent channel through the platen. A transparent block is flexibly attached to the top surface of the platen over the platen aperture to rotate with the platen. A polishing pad is provided on the top surface of the platen. This polishing pad has a planar bottom surface and a hole extending through the pad. The hole is configured to fit over the transparent block.
One of the advantages of the arrangement of the present invention is that a polishing pad may be provide that is relatively inexpensive to manufacture since the transparent block does not form part of the pad. Instead, a conventional polishing pad may be modified by cutting a hole through the pad, as long as its hole is configured to fit over the transparent block. Since the transparent block is attached to the top surface of the platen, and remains with the platen, relatively inexpensive polishing pads that do not include a transparent block may be used. Another advantage of the present invention is that provided by the flexible attachment of the transparent block to the top surface of the platen. The transparent block may thus move downwardly when the wafer is placed against the emulsion pad. In other words, the transparent block may move downwardly simultaneously with the compressing of the polishing pad under the force exerted by the wafer during a polishing operation. Hence, while saving money by using relatively inexpensive polishing pads, the present invention provides a transparent block that has a top surface that may be co-planar with the top surface of the polishing pad, since the transparent block may move downwardly due to the flexible attachment.
The earlier stated needs are also met by another embodiment of the present invention which provides a planar for a chemical mechanical polishing apparatus. The platen comprises a plainer top surface configured for supporting a polishing pad and a bottom surface. An aperture extends through the platen to provide a transparent channel through the platen between the bottom surface and the top surface. The transparent block is flexibly attached independently of the polishing pad to the top surface of the platen over the platen aperture to rotate with the platen.
The earlier stated needs are also met by another embodiment of the present invention which provides a polishing pad for a chemical mechanical polishing apparatus that has a rotatable platen with an aperture and a transparent block attached by a flexible coupling to the platen over the aperture. The polishing pad comprises a planar bottom surface preceding the polishing pad on a rotatable platen. The planar top surface forms a polishing surface for polishing a workpiece. A hole is provided which extends through the polishing pad and opens at the top surface and the bottom surface. The whole has a first portion which is configured to fit snugly around a flexible coupling, as well as a second portion configured to fit snugly around the transparent block.
One of the advantages of the polishing pad of the present invention is its relative low cost, but at the same time, however, the polishing pad may be used in a system that employs laser interferometry in a chemical mechanical polishing apparatus to measure the polishing conditions of a wafer. The polishing pad is especially adapted for fitting snugly around a flexible coupling and around a transparent block.
The earlier stated needs are also met by another embodiment of the present invention which provides a method for forming a platen and polishing pad arrangement for chemical mechanical polishing apparatus. In this method, a transparent block is flexibly attached to a rotatable platen over an aperture of the platen to form a straight transparent channel through the platen and the block. Subsequently, a polishing pad is fitted over the transparent block and onto the platen such that the transparent channel extends completely through the polishing pad.